1. Field of the Invention
The present invention relates to a magnetoresistive element and a method of manufacturing the same and a magnetoresistive element aggregate used for manufacturing the magnetoresistive element, and to a thin-film magnetic head, a head gimbal assembly, a head arm assembly and a magnetic disk drive each of which incorporates the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect, and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements. Recently, developments have been made for read heads using TMR elements to adapt to further improvements in areal recording density.
Typically, a spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a ferromagnetic layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (that is hereinafter called a sense current) is fed in the direction parallel to the plane of each layer making up the GMR element. Such a structure is called a current-in-plane (CIP) structure. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element.
In a read head having the CIP structure, the CIP-GMR element is disposed between two shield layers made of soft magnetic metal films and disposed on the top and bottom of the CIP-GMR element. A shield gap film made of an insulating film is disposed between the CIP-GMR element and the respective shield layers. In this read head the linear recording density is determined by the space between the two shield layers (hereinafter called a read gap length).
With an increase in recording density, there have been increasing demands for reductions in read gap length and track width. In read heads a reduction in track width is achieved by a reduction in width of the MR element. As the width of the MR element is reduced, the length of the MR element taken in a direction orthogonal to the medium facing surface of the thin-film magnetic head is also reduced. As a result, the areas of the bottom and top surfaces of the MR element decrease.
In the read head having the CIP structure, since the CIP-GMR element is isolated from the shield layers by the respective shield gap films, the heat release efficiency decreases if the areas of the bottom and top surfaces of the MR element decrease. Consequently, there arises a problem that, in this type of read head, the operating current is limited to secure the reliability.
On the other hand, as disclosed in JP 9-288807A, for example, there have also been proposed GMR heads each having a structure in which a sense current is fed in a direction intersecting the plane of each layer making up the GMR element, such as the direction perpendicular to the plane of each layer making up the GMR element. Such a structure is called a current-perpendicular-to-plane (CPP) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element.
A read head having the CPP structure requires no shield gap film, wherein electrode layers touch the bottom and top surfaces of the CPP-GMR element, respectively. The electrode layers may also function as the shield layers. The read head having the CPP structure is capable of solving the problem of the read head having the CIP structure previously described. That is, the read head having the CPP structure exhibits a good heat release efficiency since the electrode layers touch the bottom and top surfaces of the CPP-GMR element, respectively. It is therefore possible to increase the operating current of this read head. Furthermore, in the read head, the smaller the areas of the bottom and top surfaces of the CPP-GMR element, the higher is the resistance of the element and the greater is the magnetoresistance change amount. It is therefore possible to reduce the track width of the read head. Furthermore, it is also possible to reduce the read gap length of the read head. The foregoing concludes that the CPP structure is a technique requisite for achieving an areal recording density greater than 200 gigabits per square inches.
A read head incorporating the TMR element mentioned previously has the CPP structure, too. Typically, the TMR element incorporates: a tunnel barrier layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the tunnel barrier layer; a pinned layer disposed adjacent to the other of the surfaces of the tunnel barrier layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the tunnel barrier layer. The tunnel barrier layer is a nonmagnetic insulating layer that allows electrons to pass therethrough while maintaining the spin by means of the tunnel effect. The free layer, the pinned layer and the antiferromagnetic layer of the TMR element are the same as those of a spin-valve GMR element.
A practical CPP-GMR element is disclosed in Nagasaka et al., “Giant Magnetoresistance Properties of Spin Valve Films in Current-perpendicular-to-plane Geometry”, Journal of the Magnetics Society of Japan, vol. 25, no. 4-2, pp. 807-810, 2001. The configuration of films of a sample called S-1 listed on Table 1 of Nagasaka et al. will now be described as an example of configuration of films of the CPP-GMR element disclosed in this article. Sample S-1 has a film configuration of a single spin-valve type, incorporating a free layer, a nonmagnetic conductive layer, a pinned layer and an antiferromagnetic layer that are stacked in this order on a lower electrode. An upper electrode is disposed on the antiferromagnetic layer. The free layer is formed by stacking an NiFe layer and a CoFeB layer. The nonmagnetic conductive layer is made of Cu. The pinned layer is formed by stacking a CoFeB layer, an Ru layer and a CoFeB layer. The antiferromagnetic layer is made of PdPtMn. According to Table 2 of Nagasaka et al., the magnetoresistance change ratio (hereinafter called MR ratio), which is a ratio of magnetoresistance change with respect to the resistance, of sample S-1 is approximately 1.16 percent. Considering practical utilization of the read head, this value of MR ratio is insufficient since it is impossible to increase the output of the read head.
Nagasaka at al. also disclose the film configuration of a dual spin-valve type. With this film configuration, it is possible to make the MR ratio higher, compared with the film configuration of the single spin-valve type. However, the film configuration of the dual spin-valve type has a problem that the read gap length increases.
It is assumed that the reason why the MR ratio of conventional CPP-GMR elements such as the above-mentioned element is low is that the spin polarization of an Fe-based or Co-based material used as the material of the pinned layer and the free layer is low.
To increase the MR ratio, it has been proposed recently to employ CPP-GMR elements in which a half metal whose spin polarization is nearly 1 is used as the material of ferromagnetic films included in the pinned layer and the free layer. Typically, the MR ratio and the magnetoresistance change amount of a CPP-GMR element increase with increasing spin polarization of the ferromagnetic films included in the pinned layer and the free layer. It is assumed that the reason is as follows. If the spin polarization of a ferromagnetic film increases, the density of states in a neighborhood of Fermi energy of one spin increases in the ferromagnetic film while the density of states in a neighborhood of Fermi energy of the other spin decreases. As a result, it is assumed that the difference between the mean free path of conduction electrons of the one spin and the mean free path of conduction electrons of the other spin increases, and the MR ratio and the magnetoresistance change amount thereby increase. The spin polarization of ferromagnetic metal films included in the pinned layer and the free layer of a conventional CPP-GMR element is about 0.3 to 0.5, which is much smaller than the spin polarization of a half metal, which is nearly 1.
A CPP-GMR element using a Heusler alloy, which is a type of half metal, is disclosed in Journal of Magnetism and Magnetic Materials, vols. 198-199, pp. 55-57, Jun. 1, 1999. The MR ratio of the CPP-GMR element disclosed in this article is about 8 percent at 4.2 K.
The Heusler alloy will now be briefly described. The Heusler alloy is a term generally used for ordered alloys having a chemical composition of XYZ or X2YZ. An ordered alloy having a chemical composition of XYZ is called a half Heusler alloy. An ordered alloy having a chemical composition of X2YZ is called a full Heusler alloy. Here, X is an element selected from the group consisting of the transition metals of the Fe family, the Co family, the Ni family and the Cu family of the periodic table, and the noble metals. Y is at least one element selected from the group consisting of Fe and the transition metals of the Ti family, the V family, the Cr family and the Mn family of the periodic table. Z is at least one element selected from the group consisting of the typical elements of the periods from the third to fifth periods inclusive of the periodic table. It is assumed that emergence of ferromagnetism in a Heusler alloy results from the RKKY interaction due to an orderly arrangement of magnetic moments of the element Y.
JP 2003-218428A discloses a CPP-GMR element in which at least one of the pinned layer and the free layer includes a Heusler alloy layer made of Co2MnZ, where Z is one or two elements selected from the group consisting of Al, Si, Ga, Ge and Sn.
JP 2005-116703A discloses a CPP-GMR element in which at least one of the pinned layer and the free layer includes a Heusler alloy layer. JP 2005-116703A discloses that the mean crystal grain diameter taken in the direction parallel to the plane of the Heusler alloy layer is preferably 50 Å (5 nm) or greater, and more preferably 100 Å (10 nm) or greater. In addition, JP 2005-116703A discloses an example in which, after the Heusler alloy layer is formed, heat treatment is performed at 290° C. for four hours to make the Heusler alloy layer form a superlattice.
As disclosed in JP 2005-116703A, for TMR elements, it is also expected that a high MR ratio will be achieved by employing a Heusler alloy as the material of ferromagnetic films included in the pinned layer and/or the free layer.
JP 8-045032A discloses a method of manufacturing a spin-valve GMR element that will now be described. In the method, a spin-valve film is patterned to have a width corresponding to the track width, and a ferromagnetic layer and a layer that exhibits antiferromagnetism when heated are stacked on both sides of the spin-valve film, the sides being opposed to each other in the direction of track width, which is followed by heating the layer that exhibits antiferromagnetism when heated up to a temperature of about 240° C. so as to be transferred to be antiferromagnetic.
An MR element including a Heusler alloy layer has a benefit that it is possible to obtain a high MR ratio resulting from a high spin polarization of the Heusler alloy layer but also has a problem that variations in characteristics such as the MR ratio are great.
Reference is now made to FIG. 27(a) to FIG. 27(d) to describe the reason why such variations in characteristics occur. Typically, a Heusler alloy layer is formed by making a film to be the Heusler alloy layer and then performing heat treatment on this film to change the crystal structure of the film into a structure that achieves a high spin polarization. However, in this heat treatment process, a plurality of crystal grains in the film also grow. FIG. 27(a) schematically illustrates the state of crystal grains in the top surface of a film to be a Heusler alloy layer immediately after the film is formed. FIG. 27(b) schematically illustrates the state of the crystal grains in the top surface of the film after the heat treatment. As shown in FIG. 27(a) and FIG. 27(b), variations in size of the crystal grains in the film after the heat treatment are greater than variations in size of the crystal grains in the film immediately after the film is formed. A film having a thickness of 100 nm to be a Heusler alloy layer was actually formed and heat treatment was given to this film. As a result, it was confirmed that the diameters of typical crystal grains in the film after the heat treatment were distributed in a wide range of about 5 to 200 nm.
An MR element is fabricated by forming a layered film that will be patterned later to be the MR element, and then patterning this layered film. In the case of fabricating an MR element including a Heusler alloy layer, the layered film before undergoing patterning includes a film to be the Heusler alloy layer. On this film, heat treatment is performed after the layered film is formed, for example. In this case, in the layered film before undergoing patterning, the film to be the Heusler alloy layer is in the state illustrated in FIG. 27(b). When the layered film is patterned, the film to be the Heusler alloy layer shown in FIG. 27(b) is pattered to have the shape as shown in FIG. 27(c) and FIG. 27(d), for example. The film thus patterned is the Heusler alloy layer. Here, as shown in FIG. 27(b), since variations in size of crystal grains in the film after the heat treatment are great, there also occur variations in the number and size of crystal grains in the Heusler alloy layer obtained by patterning the film, as shown in FIG. 27(c) and FIG. 27(d). In the example shown in FIG. 27(a) to FIG. 27(d), the Heusler alloy layer of FIG. 27(d) has a greater number of small crystal grains than the Heusler alloy layer of FIG. 27(c), and the sum of surface areas of the grain boundaries are therefore greater in the Heusler alloy layer of FIG. 27(d). In this case, an MR element including the Heusler alloy layer of FIG. 27(d) has a lower MR ratio, compared with an MR element including the Heusler alloy layer of FIG. 27(c). As thus described, it is assumed that variations in the number and size of crystal grains in a Heusler alloy layer are the cause of variations in characteristics of an MR element including the Heusler alloy layer.